The rollers generally in use in the printing industry today are metal roller cores covered with either a solid rubber or a solid plastic cover of varying degrees of hardness.
The solid rubber or plastic covered rollers are elastic but they are not compressible. As a result, when pressures are applied to such rollers, the rubber or plastic moves away from the point of application of pressure in several directions causing distortions or deformation of the rollers without substantially changing their volume.
In most printing operations, a printing roller is used in conjunction with another roller, generally a steel roller. When pressure is exerted on a printing roller with a solid rubber or solid plastic cover by a steel roller, the incompressibility of the cover results in ripples or bulges being formed at the nip and a change in the circumference of the printing roller. In addition to distorting the image being printed, this change in circumference can result in changes of the velocity of the printing roller resulting in heat buildup that can cause the premature destruction of the printing roller, the generation of static electricity and other undesirable effects.
In the Ross U.S. Pat. No. 3,467,009, the various disadvantages of non-compressible solid rubber or plastic covered printing rollers and the advantages of having a compressible printing roller are described. The Ross patent's answer to the need for a compressible printing roller is a metal roller core with a cover having at least three distinct layers of different materials. The first innermost layer of the cover is a layer of highly flexible sponge rubber which covers the metal core. The intermediate layer(s) is (are) of a compressible, dimensionally stable material which is disclosed in the patent as being a fibrous web impregnated with rubber. The outermost layer is a thin protective coating of rubber which covers the outer surface of the intermediate layer(s) and provides a printing surface.
Although the Ross patent roller is described for use in the printing industry, it is not known to have been used therein. Perhaps, the reason is that the Ross patent roller has only a limited ability to recover to its original thickness immediately upon release of an applied force and that is not adequate for use in today's high speed printing systems. In addition, when used in high speed printing the Ross roller construction because of its three or more distinct layers of different materials, is likely to generate excessive internal heat which can cause the premature destruction of the roller.
In the Pinkston et al U.S. Pat. No. 4,378,622, a compressible printing roller is described which consists of a rigid core, a tubular inner layer of microporous, open celled rubber material and a tubular cover disposed about the inner layer. The cover is made of a non-porous polymeric material which provides a printing surface. The inner and outer layers of the Pinkston et al roller are preferably frictionally bound to each other; however, the optional use of an adhesive for that purpose is mentioned. The Pinkston et al roller appears to be relatively difficult to assemble, which probably accounts for it apparently not being used in the printing industry.
Neither the Ross or the Pinkston et al rollers appears to be without serious disadvantages. Therefore, a need still exists for an inexpensive, compressible roller for use in the printing industry.